Low Power Modes for Data Transmission from a Distribution Point

ABSTRACT

Methods and devices are discussed where a common bit loading table is constructed from minimum gain from a plurality of bit loading tables for different combinations of lines being in a transmit or quiet mode.

TECHNICAL FIELD

The present application relates to low power modes for data transmissionfrom a distribution point.

BACKGROUND

Recent trends in the access communications market show that data ratesup to 100 Mb/s which are provided by VDSL systems using Vectoring asdefined in ITU-T Recommendation G.993.5 are not sufficient for allapplications and bit rates up to 1.0 Gb/s are required in some cases. Toachieve these targets, for wire-based implementations currently copperpairs connecting the CPE must be as short as 50-100 m. Operation usingso short loops requires installation of many small street/MDU (MultiDwelling Unit) cabinets called Distribution Points (DP) that serve avery small number of customers, e. g. 16 or 24 and are connected to thebackbone via fiber (fiber to the distribution point FTTdp).

Vectoring is used in systems operating from a DP [GC02], to reducefar-end crosstalk (FEXT), which is absolutely necessary to obtain highbit rates. To improve energy efficiency and to reduce hardwarecomplexity, synchronized time division duplexing (S-TDD) is used forFTTdp instead of frequency division duplexing (FDD) which is used inVDSL.

DPs shall allow very flexible installation practices: they should belight and easy to install on a pole or house wall, or basement, withoutair-conditioning. The most challenging issue for these flexibleconnection plans is providing DPs with power. The only solution found isso-called “reverse feeding” when the equipment of the DP is fed by theconnected customer. The requirement of reverse power feeding and thesmall size of the DP implies substantial restrictions on the powerconsumption of the DP, which main contributors are DSL transceivers.Further, the customer unit is often required battery-powered operation(to support life line POTS during power outages). The latter applies lowpower requirements also to DSL transceivers of the CP equipment.

Conventional DSL systems transmit data continuously on all lines sharinga cable binder. Whenever there is no data available, idle bytes aretransmitted. With this type of static operation, the system stabilityand performance is maintained.

In current DSL systems (e.g., ADSL), low-power modes and data rateadaptation use a method that reduces bit loading and TX (Transmit) poweron the line when data traffic turns to be slow and reconstructs it backwhen high speed traffic is back. Other proposed methods use called SRA(Seamless Rate Adaptation) to reconfigure the bit rate and TX power ofthe links. Both reconfiguration methods are too slow to perform adaptivelink reconfiguration with respect to the actual traffic requirements ofthe subscribers.

Also in terms of power saving, current DSL transceivers only allow powersaving by transmit power reduction. The transmit power in VDSL systemsis in the range of 14 dBm to 20 dBm and therefore, the transmit powerlargely contributes to the overall power consumption.

However in FTTdp applications, the transmit power is only a smallportion of the overall power consumption, because the aggregate transmitpower is in the range of 4 dBm. Components like the analog and digitalfrontend electronics consume power irrespective of the transmit power,but these components significantly contribute to the overall powerconsumption, because they operate at much higher frequencies of 100 MHzor 200 MHz in comparison to 8 MHz-30 MHz in VDSL.

Therefore, to provide significant power savings also analog and digitalcomponents of the transceiver, such as analog front end (AFE) anddigital front end (DFE), need to be switched into low-power (standby)state. This operation mode is called Discontinuous Operation.

In currently operated systems using Vectoring, such as ITU G.993.5, atime consuming procedure called “orderly leaving” is required before alink can be switched off. If a line is disconnected without orderlyleaving, the remaining active lines of the binder experience substantialperformance drops. Therefore, AFE and DFE cannot be turned off for shorttime which substantially reduces power savings.

Therefore, improvements in using low power modes like discontinuousoperations together with vectoring would be desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a communication system accordingto an embodiment.

FIG. 2 illustrates minimum transmit power spectral densities (PSDs)satisfying a spectral mask constraint for all configurations.

FIG. 3 illustrates a comparison between peak rates with and withoutdiscontinuous operation with bit loading and PSD downscaling.

FIG. 4 illustrates an example for twotime division duplex (TDD) frameswith discontinuous operation.

FIG. 5 illustrates simulation results for an example system showingassignment of data rates within a TDD frame for a minimum configuration,where an overall on-time is 50%.

FIG. 6 illustrates the final step of a line joining sequence.

FIG. 7 illustrates a flowchart according to an embodiment illustratingline joining with discontinuous operation.

FIG. 8 illustrates a downstream model with a linear precoder.

FIG. 9 illustrates a drop of signal to noise ratio (SNR) of active lineswhen other lines are discontinued.

FIG. 10 illustrates an example for an effect of coefficientrecalculation on a signal to noise ratio (SNR) of active lines during alow power mode.

FIG. 11 illustrates an upstream model with a linear equalizer.

DETAILED DESCRIPTION

Embodiments will be described in the following in detail with referenceto the attached drawings. It should be noted that these embodimentsserve as illustrative examples only and are not to be construed aslimiting. For example, while embodiments may be described havingnumerous details, features or elements, in other embodiments some ofthese details, features or elements may be omitted and/or may bereplaced by alternative features or elements. In other embodiments,additionally or alternatively further features, details or elementsapart from the ones explicitly described may be provided.

Communication connections discussed in the following may be directconnections or indirect connections, i.e. connections with or withoutadditional intervening elements, as long as the general function of theconnection, for example to transmit a certain kind of signal, ispreserved. Connections may be wireless connections or wire-basedconnections unless noted otherwise.

In some embodiments, a low power mode using discontinuous operation isprovided.

In some embodiments, the discontinuous operation is used in a vectoredsystem. In some embodiments, mechanisms for joining of lines to avectored group may be provided.

Turning now to the figures, in FIG. 1 a communication system accordingto an embodiment is shown. The system of FIG. 1 comprises a providerequipment 10 communicating with a plurality of CPE units 14-16. Whilethree CPE units 14-16 are shown in FIG. 1, this serves merely as anexample, and any number of CPE units may be provided. Provider equipment10 may be central office equipment, equipment in a distribution point(DP), or any other equipment used on a provider side. In case providerequipment 10 is part of a distribution point, it may for example receiveand send data from and to a network via a fiber optic connection 110. Inother embodiments, other kinds of connections may be used.

In the embodiment of FIG. 1, provider equipment 10 comprises a pluralityof transceivers 11-13 to communicate with CPE units 14-16 via respectivecommunication connections 17-19. Communication connections 17-19 may forexample be copper lines, e.g. twisted pairs of copper lines.Communication via communication connections 17-19 may be a communicationbased on a multicarrier modulation like discrete multitone modulation(DMT) and/or orthogonal frequency division multiplexing (OFDM), forexample a xDSL communication like ADSL, VDSL, VDSL2, G.fast etc., i.e. acommunication where data is modulated on a plurality of carriers, alsoreferred to as tones. In some embodiments, the communication system mayuse vectoring, as indicated by a block 111 in FIG. 1. Vectoringcomprises joint processing of signals to be sent and/or received toreduce crosstalk.

A communication direction from provider equipment 10 to CPE units 14-16will also be referred to as downstream direction, and a communicationdirection from CPE units 14-16 will be also be referred to as upstreamdirection. Vectoring in the downstream direction is also referred to ascrosstalk precompensation, whereas vectoring in the upstream directionis also referred to as crosstalk cancellation or equalization.

Provider equipment 10 and/or CPE units 14-16 may include furthercommunication circuits (not shown) conventionally employed incommunication systems, for example circuitry for modulating, bitloading, Fourier transformation etc.

In some embodiments, communication via communication connections 17-19is a frame-based communication. A plurality of frames may form asuperframe.

In some embodiments, discontinuous operation is employed. However, aconventional application of discontinuous operation as discussed furtherbelow to reduce power consumption may include some unsolved problems insome implementations.

A problem in conventional approaches is that if no precoder (downstream)and equalizer (upstream) coefficient recalculation is performed, the SNRin discontinuous operation drops. With coefficient recalculation, thetransmit power (downstream) is increased and may violate the limit. Inupstream, the noise power is increased and causes a change of the SNR.In this respect, precoder and equalizer refer to elements used invectoring (crosstalk reduction through joint processing), the precoderfor vectoring in the downstream direction and the equalizer forvectoring in the upstream direction. The coefficients mentioned aboveare coefficients employed in the precoder or equalizer, respectively.Further information regarding these issues will be given further below.

A working solution in some embodiments requires that no errors orsubstantial noise margin drop occur as well as transmit power spectraldensity (PSD) are maintained to guarantee stable operation in everyparticular configuration of active and discontinued lines in a givendeployment, so that no violation in PSD; power,bit loading or SNR occurin any configuration.

The problem can be solved by finding a single configuration of transmitpowers and bit loadings that takes into account the transmit powervariantes and SNR variations of discontinuous operation but does notviolate the constraints for any configuration.

The methods used to find this configuration in an optimized way requirespecific information from the CPE side to be communicated the DP, whichperforms the optimization.

The invention shows how to find this configuration and how toincorporate the corresponding operations into system initialization andline joining process.

The invention provides method of selection this configuration thatresult in minimum performance degradation.

To maintain stability of the system operating in low power mode andsatisfy the spectral mask constraints, some embodiments includesearching a minimum configuration (of transmit PSD and bit loading)which works for all cases of active and discontinued lines.

To find this stable configuration, we define a set of active lines I_(a)⊂ {1 . . . L} which is a subset of all lines. Furthermore, we define aset of all configurations T={I_(a 1), . . . , I_(a t), . . . I_(a T)}which contains all possible sets of active lines I_(a 1) . . . I_(a T)of all available configurations t=1 . . . T.

For a system with L lines, there are 2^(L) possible configurations,which means that the cardinality of the set of configurations T is|T|=2^(L) which might be a very high number. Some embodiments offermethods to reduce the associated computation complexity.

In downstream direction, a scale matrix S_(min) is searched, which is asingle scaling matrix that satisfies the transmit power constraints forall possible configurations. The scale matrix S_(t) for configuration t∈ T satisfies Eq. (1.13) for the precoder matrix as given by Eq. (1.6).

The transmit power which satisfies the transmit power constraint for allcases is then given by

$\begin{matrix}{s_{\min \; {ii}} = {\min\limits_{t = {1\ldots \; T}}s_{{ii}\; t}}} & (3.1)\end{matrix}$

where s_(ii t) is the scale factor satisfying Eq. (1.14) or (1.20) forthe corresponding subset of active lines I_(a t).

FIG. 2 shows mean and maximum PSDs for the lines from an example of Tab.1.1 (found further below) for different cases.

In embodiments, the maximum PSD is calculated according to

${\max \; {PSD}} = {\max\limits_{t \in }\left( {\max\limits_{l \in _{at}}c_{txllt}} \right)}$

from the precoder output transmit covariance matrix according to Eq.(1.12).

The mean PSD is given by mean

${PSD} = {\frac{1}{_{a}}{\left( {\sum\limits_{l \in _{a}}\; c_{txll}} \right).}}$

Simulation results for 10 lines of 100 m length each, with all possibleconfigurations searched through are shown in FIG. 2.

The maximum PSD without discontinuous operation matches the limit PSD,as shown by the light blue line.

The red line shows that if coefficient recalculation is applied withoutany further PSD correction, the transmit PSD will violate the limit PSD.

The light green line shows the maximum PSD of the minimum configuration.It is below the limit, but still very close to the limit.

The mean PSDs of the minimum configuration compared to the mean PSDwithout discontinuous operation show that the additional PSD reductionrequired for the minimum configuration is close to zero for lowerfrequencies and is up to 2 dB at higher frequencies.

As a result of PSD reduction at higher frequencies, the minimumconfiguration results in a performance degradation compared to the casewithout discontinuous operation. For the given example of 10 lines thedata rate reduction caused by spectrum minimization is shown in FIG. 3.

With the aim to select the minimum configuration, the DP needs also tocompute the required bit loading associated with minimum configuration.For this, a feedback from the CPE of each line is required to deliversensitive information, such as measured SNR or channel-relatedparameters, such as FEQ coefficients or obtained FFT samples. Thoseparameters are obtained by the joining line during initialization, asdescribed further below in more detail.

In uplink (upstream) direction, a similar problem may occur becausethere, the coefficient recalculation according to Eq. (1.10) results ina change of the receiver noise covariance which causes a change of thesignal-to-noise ratio SNR.

The transmitted bits b₁ on a particular subcarrier of line 1 depend onthe SNR according to Eq.:

$\begin{matrix}{b_{l} = {\left\lfloor {\log_{2}\left( {1 + \frac{SNR}{\Gamma}} \right)} \right\rfloor.}} & (3.2)\end{matrix}$

Therefore, the bit loading in uplink direction must be selected withrespect to the worst case SNR

$\begin{matrix}{b_{i\; \min} = {\min\limits_{t = {1\; \ldots \; T}}{b_{it}.}}} & (3.3)\end{matrix}$

FIG. 4 shows an example of a transmission frame (TDD frame) of a systemapplying discontinuous operation with minimum PSD and bit loading.

It must be noted that in some embodiments, the PSD and bit loadingsatisfying the constraints for a single configuration may be higher thanthe bit loading and PSD of the case of all lines active.

In some cases, the minimum configuration which consists of the minimumgain factors (Eq. (3.1)) in downstream and the minimum bit loading (Eq.(3.3)) in upstream is dominated by a single configuration whichsignificantly reduces the overall performance, also referred to ascritical configuration.

In cases where the search of the minimum configuration indicates acritical configuration, some embodiments avoid this configuration. Suchcritical configurations are excluded form the set T of the availableconfigurations. The set of critical configurations is stored.

If the critical configuration of enabled and disabled lines occursduring data transmission, the corresponding lines are not switched off,but they transmit idle symbols, instead, to avoid that the correspondinganalog and digital front-ends are discontinued. The idle symbols may betransmitted with zero power to reduce power consumption during idlesymbol transmission.

Next, some simulation results for a non-limiting example system will bediscussed. The example system consists of 10 lines with 100 m lengtheach. The target rates are set to 800 Mbit/s for lines 1 and 2, 100Mbit/s for lines 3 to 6 and 500 Mbit/s for lines 7 to 10. FIG. 3.4 showsthe scheduling for a TDD frame with 40 DMT symbols. The average on-timeof the links to achieve this data rates is 50%. One of the lines usesalmost the complete TDD frame because it is close to its peak data rate.The data rates of the links are constant over the frame, because thesame bit loading is used for all symbols.

In FIG. 5 the symbols of active data transmission are assigned such thatsome lines do not start transmission at the start of the TDD frame, butwith some delay in the middle of the frame.

The transmission times may also be assigned such that all lines start totransmit at the start of frame and transmit the number of symbolsrequired to reach the target data rate. This method simplifies thecommunication overhead for discontinuous operation, because then onlythe end of transmission must be communicated from transmit side toreceive side while the start of transmission is fixed.

But due to limitations in the crosstalk cancelation capabilities orcoefficient recalculation speed, the delayed start of transmission asshown in the simulation may be required in some cases.

The above-discussed methods for discontinuous operation may befacilitated during initialization in some embodiments. The line joiningor system activation procedure contains multiple steps. Variousstandards and standard proposals, e.g. for G.fast, describe a possibleinitialization procedure in detail. It may contain many steps forchannel estimation, synchronization, setting transmit PSD levels andother tasks. For discontinuous operation, the associated and criticalstep is the transmit PSD optimization before showtime, as shown in FIG.6.

The DP calculates precoder and equalizer coefficients based on syncsymbols. They are transmitted once per superframe and are not subject ofdiscontinuous operation.

Therefore, the precoder and equalizer coefficients after the linejoining are calculated for a configuration in which all lines are activein some embodiments.

We assume a set of active lines I_(a) with |I_(a)|=L_(a) lines and a setof joining lines I_(j) containing |I_(j)|=L_(j) lines. With thisassumption, we show how to apply minimum configuration in embodiments.

Next, joining of lines with minimum configuration will be discussed.

The search of the minimum configuration in embodiments takes place atthe spectrum optimization step of the joining sequence as shown in FIG.6. The required information includes the full precoder and equalizercoefficient matrices (including both, active and joining lines) and aSNR estimation from the joining and active lines. This SNR estimation islocal for the case of upstream and shall be provided by each of the CPEsfor the case of downstream. FIG. 7 summarizes steps to be done fortransmit spectrum optimization with discontinuous operation in someembodiments.

The downstream SNR data is provided by the CP by sending correspondingmessages from the CPE to the DP. It shall be available for thecomputation of new Scaling S_(t) ^((n)) in FIG. 7.

It must be noted that the precoder coefficients after line joiningrequire a re-scaling of the precoder matrix to comply with Eq. (1.17)which guarantees that the diagonal elements are equal to 1 to match thedefinition in Eq (1.2). The inverse of the scaling must be multiplied tothe scale matrix S_(a) to make sure that it matches Eq. 3.1 for theactive lines.

The exhaustive search over all 2^(L) possible configurations which isnecessary for (3.1) can be reduced to

$\begin{matrix}{s_{ii} = {\min \left( {s_{aii},{\min\limits_{{t \in }{r \notin _{a}}}\left( s_{iit} \right)}} \right)}} & (3.4)\end{matrix}$

which means that e. g. for a system with 9 active lines and the 10thline joining, only 512 (2^(L)−2^(L) ^(a) ) instead of 1024 (2^(L))configurations must be searched.

Nevertheless, the search over all possible configurations may be complexand might be simplified by limiting the search to criticalconfigurations, for example the configurations where only one lineleaves and the configurations where only two lines are left active.

Next, protocol additions for discontinuous operation according to someembodiments will be discussed.

To implement the described techniques, an exchange of informationbetween DP and CPE is required in some embodiments. This sectiondescribes the details of the protocols for some embodiments. Similar orrelated information exchange may be used in other embodiments.

Next, protocols for discontinuous operation and line joining accordingto embodiments will be discussed.

During joining of new lines, additional resources are allocated to servethe joining lines by reducing the transmit PSDs of the active lines.FIG. 6 shows the initialization sequence which is used to start a lineof a vectored group or to join a new line to the operating vectoredgroup. When new lines join, the size of the precoder matrices(downstream) and equalizer matrices (upstream) is extended in two steps.First, the coupling paths from joining lines into active lines areestimated and the corresponding crosstalk is canceled and then, thecrosstalk from the active lines into the joining lines is canceled.

The size of the precoder matrices (downstream) and equalizer matrices(upstream) is extended in two steps. First, the coupling paths fromjoining lines to active lines are estimated and the correspondingcrosstalk is canceled and then, the crosstalk from the active lines intothe joining lines is canceled.

The optimization of the transmit spectrum and setting final transmitPSDs follows crosstalk cancellation steps and is very time consuming. Italso requires SNR estimation from the joining lines with canceledcrosstalk and active lines. Therefore in majority of implementations itis only done once, at the step of the initialization sequence duringwhich final transmit PSD value is set. During the joining process,discontinuous operation in all lines is limited, which means that thefrontends of all lines are kept active or switched of jointly.

Next, a bit loading protocol in downstream direction will be discussed.

In conventional systems like VDSL, the receive side monitors the SNR anddetermined a specific bit loading with respect to the measured SNR. Dueto the fact that the SNR in discontinuous operation depends on theapplied configuration of active and discontinued lines, the bit loadingshall be selected with respect to the configuration representing theworst case SNR. When the minimum configuration is used as describedabove, this is the setting when all lines are active. A symbol which isguaranteed to be transmitted with this configuration is the SYNC symbol.Therefore, in one embodiment SNR measurement shall be done during syncsymbols.

Next, a bit loading protocol in upstream direction will be discussed.

In upstream, the DP in embodiments is able to assign additional SNRmargin on specific lines and subcarriers to maintain stability. If SNRis evaluated only on SYNC symbols, as for downstream.

The noise covariance matrix C_(rx) after a vector equalizer is given by

C _(rx) =G·(σ_(noise) ² ·I)·G ^(H)   (3.5)

with the noise covariance matrix (σ_(noise) ²·I) and G being anequalizer matrix.

The frequency dependent additional SNR margin required in upstream isgiven by

$\begin{matrix}{{\Gamma_{DO}(f)} = {\Gamma \frac{{diag}\left( {\max_{t = {1\ldots \; T}}{C_{rxt}(f)}} \right)}{{diag}\left( {C_{{rx}\; {all}\; {active}}(f)} \right)}}} & (3.6)\end{matrix}$

The relative margin is evaluated once during joining, but it may beupdated in Showtime due to changes in the coefficient matrix or thereceiver noise.

Next, frequency equalization (FEQ) and noise environment will bediscussed.

For the implementation of downstream spectrum optimization as describedfurther below, information from the CPEs is needed to estimate datarates. The required information from the CPE side is the SNR obtainedfor the crosstalk free case (or for case the crosstalk is substantiallycancelled). This information is required at the spectrum optimizationstep of the initialization sequence shown in FIG. 6. A number ofembodiments using different methods and approaches are presented next.

Method 1: SINR

One method is to measure signal-to-noise ratio at CPE side by detectingthe average error power. The signal-to-interference-noise-ratio (SINR)SINR_(i) of line i in downlink direction is given by

$\begin{matrix}{{SINR}_{i} = \frac{\sum\limits_{t = 1}^{T}\; {{\hat{u}}_{i\; t}}^{2}}{\sum\limits_{t = 1}^{T}\; {e_{i\; t}}^{2}}} & (3.7)\end{matrix}$

where the error e is defined as

e _(i t) =û _(i t) −u _(i t).   (3.8)

T is the number of symbols used for averaging, which is selectedsufficiently large. The drawback of this method is that the receivererror e may contain residual crosstalk due to limitations of thecrosstalk canceller or just because the crosstalk canceller coefficientsare not fully converged. The error e is usually calculated as thedifference between the receive signal and the closest constellationpoint of the receive signal. If the closest constellation point is notequal to the transmitted data, the error is not calculated correctly.

Method 2: SNR Zero-State

To avoid the impact of residual crosstalk and detection errors, the SNRestimation may be limited to the sync symbols, which are modulated withan orthogonal sequence that is known to the receiver. However, eachreceiver only knows the orthogonal sequence used from the correspondingtransmitter. The orthogonal sequences used by the other transmitters arenot known and can't be used estimate the crosstalk signal from otherlines.

In some approaches, orthogonal sequences including a zero-state wereproposed. This allows the CPE side to estimate the crosstalk-free erroron line i by selecting the transmit signal u₁=0 for 1≠i and u₁≠0 for1=i. With this method, the error on line i according to Eq. 3.8 iscrosstalk-free and turns into

$\begin{matrix}{{SNR}_{i} = \frac{\sum\limits_{t = 1}^{T}\; {{\hat{u}}_{i\; t}}^{2}}{\sum\limits_{t = 1}^{T}\; {e_{i\; t}}^{2}}} & (3.9)\end{matrix}$

Method 3: SNR BPSK Sequence

If the orthogonal sequence applied to the sync symbols for channelestimation is static, it is possible to eliminate crosstalk in the SNRestimation using the orthogonal sequence. For this, the number ofsymbols T used for averaging is selected to be a multiple of thesequence length T_(seq), T=N_(seq)·T_(seq). The sequence length T_(seq)shall be selected as short as possible to have some noise in theestimation. The crosstalk-free SNR is calculated at CPE side byevaluating

$\begin{matrix}{{SNR}_{i} = {\frac{1}{T_{seq}}{\sum\limits_{n = 0}^{N_{seq} - 1}\; {\frac{{{\sum\limits_{t = {n + 1}}^{t = {n + T_{seq}}}\; {u_{it} \cdot {\hat{u}}_{it}^{*}}}}^{2}}{{{\sum\limits_{t = {n + 1}}^{t = {n + T_{seq}}}\; {u_{it} \cdot e_{it}^{*}}}}^{2}}.}}}} & (3.10)\end{matrix}$

The invention proposes to request the crosstalk-free SNR from the CPEside as basis for the spectrum optimization. This is an extension toMethod 2 and 3. The reported crosstalk-free SNR represents the term

$\frac{{\left\lbrack H^{- 1} \right\rbrack_{ll}^{- 1}}^{2}}{\sigma_{noise}^{2}}$

in Eq. (1.20).

The invention furthermore proposes to reduce time and increase precisionof required to evaluation of Eq. (3.9) or Eq. (3.10) by performing theaveraging not only over time, but also on multiple adjacent tones. Whilethe channel transfer function and the transmit PSD may vary widely fromsubcarrier to subcarrier, the noise itself is usually flat. Assumingthat the transmit signal u has unit power, averaging may be performedover a group of tones tones, the effective noise power σ_(noise) ²

$\begin{matrix}{\sigma_{noise}^{2} = {\sum\limits_{n \in {tones}}\; {\sum\limits_{t = 1}^{t = T}\; {{g_{ii}^{(n)}}^{- 2}{e_{it}^{(n)} \cdot e_{it}^{{(n)}*}}}}}} & (3.11)\end{matrix}$

can be estimated, which is flat over frequency.

The SNR, which is then given by

${SNR}_{i} = \frac{1}{{g_{ii}^{(n)}}_{noise}^{2_{\sigma}2}}$

is communicated to the DP to run the optimization.

The power consumption of a distribution point will be reducedsignificantly, if active links are disabled whenever there is no data totransmit. On a disabled link, power is not only saved by reducedtransmit power. The analog and digital components can be switched intolow-power state and consume very low power. This operation mode iscalled Discontinuous Operation.

The use of discontinuous operation in FTTdp applications causes twomajor problems.

In wireline communication each line experiences some interference fromthe data transmission on neighboring lines of the cable binder, calledcrosstalk. Each link experiences some crosstalk noise and chooses theachievable data rate according to the remaining signal-to-noise ratio.With discontinuous operation, the noise environment is no longer static,but changing very fast.

This causes the first problem which may occur, namely that the noiseenvironment is no longer time invariant and the receivers do notestimate the signal-to-noise ratio and the achievable data ratescorrectly, which may result in an increased bit error rate.

A second problem occurs in systems applying joint signal processingoperations over multiple links like Vectoring. If a transmitter indownlink direction is turned off, the precoder coefficients used beforethat time do no longer work. The same holds if a receiver in uplinkdirection is switched off.

The next section addresses the second problem. To keep the performanceof active links in a Vectoring system, while other links are switched tolow power mode, coefficient recalculation is required. For coefficientrecalculation, multiple system configurations are considered, linearprecoding in downlink direction, linear equalization in uplink directionand nonlinear precoding in downlink direction.

Crosstalk cancelation and other MIMO (multiple input multiple output)signal processing methods are an important feature to improveperformance of multi-user data transmission. Vectoring is successfullyused to improve VDSL2 performance and for future wireline communicationstandards such as G.fast, crosstalk cancelation is mandatory.

Therefore, the proposed low-power modes, e.g. as discussed above, shallbe compliant with systems using MIMO signal processing. This sectiondiscusses how to implement discontinuous operation in combination withlinear MIMO precoding and equalization which has been proposed for FTTdpapplications. Linear vector precoding has been implemented on VDSL 2systems to improve performance for wireline data transmission overcrosstalk channels. The main drawback of conventional Vectoring DSLsystems is that all links are enabled continuously and turning a link onor off requires very time-consuming joining and leaving procedures toenable or disable data transmission on a particular line of the vectoredgroup.

The downstream transmission model as shown in FIG. 8 is represented by

û=G·(H·P·S·u+n)   (1.1)

where u is the transmit signal vector of multiple parallel datatransmissions at the DP and û is the corresponding receive signal at CPEside with the noise n added at the receivers. P is the precoder matrixat DP side, H is the crosstalk channel matrix and G is a diagonal matrixof equalizer coefficients.

The precoder matrix P is normalized according to

P=H ⁻¹·diag(H ⁻¹)⁻¹   (1.2)

S is the scaling diagonal matrix to do transmit spectrum shaping. It isused to scale the transmit power relative to the limit transmit PSDmask. The linear vector precoder P performs crosstalk cancelation byprecompensation of the crosstalk. The diagonal matrix G consists of onenonzero coefficient per line and per subcarrier at the CPE side and isused to perform gain and phase correction of the direct path betweentransmitter and receiver.

In multicarrier systems, Eq. (1.1) describes the operation of onesubcarrier and each of the subcarriers are precoded independently.

For the proposed low power mode according to embodiments, some of thetransmitters shall be switched off. This corresponds to the operation ofsetting the corresponding rows and columns of the precoder matrix P tozero. If this is done without changing the coefficients of the precodermatrix

P, the remaining active lines will experience significant performancedrops as shown in FIG. 9. The simulation shows that discontinuousoperation can only be implemented for very low frequencies (where FEXTis small compared to the received signal) without coefficientcorrection. Tab. 1.1 below summarizes the simulation parameters used forthe calculation.

To cancel crosstalk between the remaining lines in this state, theprecoder coefficients for the active lines must be recomputed. For thefollowing derivation, the equalizer matrix G is neglected without lossof generality, as the equalizer matrix can be included into the channelmatrix H. Then, in the case of all lines active,

H·P−I   (1.3)

holds.

With the precoder and channel matrix rewritten as block matrices,

$\begin{matrix}{{\begin{bmatrix}H_{aa} & H_{ad} \\H_{da} & H_{dd}\end{bmatrix} \cdot \begin{bmatrix}P_{aa} & P_{ad} \\P_{da} & P_{dd}\end{bmatrix}} = \begin{bmatrix}I & 0 \\0 & I\end{bmatrix}} & (1.4)\end{matrix}$

TABLE 1.1 Parameters of simulation example Parameter Value Lines inbinder 10 Binder length 100 m Cable type BT cable Direction downlinkTransmit PSD  −76 dBm/Hz flat Noise PSD −140 dBm/Hz flat Spectrum 2MHz-106 MHz Transmit power 2 dBmholds for the linear zero-forcing precoder in case that all lines areactive. The index a denotes the active lines and the index d denotes thedisabled lines. For example, the block matrix H_(ad) contains thecouplings from the disabled to the active lines.

For the case of all lines active Eq. (1.3) must hold which can bedivided into block matrices as shown in Eq. (1.4). After turning the setd of transmitters off

H _(aa) ·P _(aa) ′=I   (1.5)

must hold, where P′_(aa) is the new precoder matrix in low power mode.The new precoder matrix for the remaining active lines is given by

P _(aa) ′=P _(aa) −P _(ad) ·P _(dd) ⁻¹ ·P _(da)   (1.6)

according to the matrix inversion lemma.

Instead of recomputation of the precoder matrix coefficients P′_(aa), itis also possible to recomputed the transmit signal during low powermode. The transmit signal vector x is given by

x=P·u.   (1.7)

Then, the transmit signal of the active lines with some linesdiscontinued is given by

x _(a) =P _(aa) u _(a) −P _(ad) −P _(dd) ⁻¹ ·P _(da) ·U _(a)   (1.8)

After precoder coefficient correction (Eq. (1.6)) or signalrecalculation (Eq. (1.8)), the SNR of the active lines is recovered whenother lines are turned off, as shown in FIG. 10.

It must be noted that any change of the precoder coefficients changesthe transmit spectrum. Therefore, the coefficient recalculation maycause violations of the transmit power constraints which require are-computation of the transmit powers. This is explained further belowin more detail.

In upstream direction, linear vector equalization is used instead oflinear precoding.

The system model is shown in FIG. 11 which corresponds to

û=G·H·S·u.   (1.9)

Similar to the downstream direction u is the transmit signal vector, ûis the receive signal vector and H is the channel matrix. At CPE side,the diagonal matrix S can be used to scale the transmit power, but withcrosstalk cancelation, this is not necessary and it may be set to S=I.Crosstalk cancelation and direct channel gain and phase correction areperformed with the equalizer matrix G, which is a full matrix inupstream case.

Similar to the downstream case, coefficient recalculation can be done by

G _(aa) ′=G _(aa) −G _(ad) ·G _(dd) ⁻¹ ·G _(da).   (1.10)

Alternatively, the recalculation based on the receive signal accordingto

û _(a) −G _(aa) y _(a) −G _(da) ·G _(dd) ⁻¹ ·G _(ad) ·y _(a)   (1.11)

can be implemented.

This recalculation changes the noise environment, because the receivesignal y consists of received signal plus noise y=H·u+n. This change innoise power leads to a SNR change which may require reducing the bitloading.

For vector precoding, nonlinear precoding with the Tomlinson

Harashima precoder is discussed as an alternative to linear precoding.In uplink direction, the Generalized Decision Feedback Equalizer (GDFE)is a possible implementation.

This modification of the precoder matrix results in a change of thetransmit spectrum, similar as in the linear precoder case. Othernonlinear precoders and equalizers may also be used.

Next, transmit spectrum shaping will be discussed.

Transmit power in wireline communication is limited by regulation andfor technical reasons. To satisfy regulatory constraints and to use theavailable transmit power as efficient as possible, transmit spectrumshaping is used.

The output spectrum of the linear precoder as well as the nonlinearprecoder is different to the input spectrum. To keep the crosstalkcancelation capabilities while changing the transmit spectrum, thetransmit spectrum is shaped at the precoder input with the scale matrixS as shown in FIG. 8. The transit covariance matrix C_(tx) is then givenby

C_(tx)=PSS^(II)P^(II),   (1.12)

where the diagonal elements correspond to the transmit power of theindividual ports. In wireline communication, the per-line transmitspectrum is constrained by a spectral mask which is equivalent to amaximum transmit power pmax

c_(tx ii)≦p_(max)   (1.13)

which in general depends on frequency. This section shows two spectrumshaping approaches for wireline communication with linear precoding indownlink direction.

A simple approach is to select the scale factors for a transmit spectrumscaling with respect to the line with the highest gain. Then, the scalefactors are given by

$\begin{matrix}{s_{ii} = {\sqrt{\frac{p_{\max}}{\max \; {{diag}\left( {PP}^{H} \right)}}}.}} & (1.14)\end{matrix}$

This spectrum scaling method guarantees that the output spectrumcomplies with the spectral mask on all lines, but only one line will beclose to the maximum, while the other lines are scaled lower than that.In general, there is no input transmit spectrum such that all lines cantransmit with maximum power. But it is possible to calculate an inputspectrum such that the data rates are maximized as shown in the nextsection.

To improve performance, transmit spectrum optimization may be applied.The data rate R₁ of link 1 for linear zero forcing precoding is given by

$\begin{matrix}{R_{l} = {{\log_{2}\left( {1 + \frac{\left\lbrack H^{- 1} \right\rbrack_{ll}^{- 1} \cdot {s_{l}}^{2}}{{\Gamma\sigma}_{noise}^{2}}} \right)}.}} & (1.15)\end{matrix}$

It depends on the channel matrix H, the scale factors S and on the noisevariance σ_(noise) ².

Equation (1.15) assumes that the SNR is given by

$\begin{matrix}{{SNR}_{l} = {+ {\frac{\left\lbrack H^{- 1} \right\rbrack_{ll}^{- 1} \cdot {s_{l}}^{2}}{\sigma}\;}_{noise}^{2}}} & (1.16)\end{matrix}$

as a function of the channel matrix H, the receiver noise powerσ_(noise) ² and the scale matrix S. This holds for a linear zero forcingprecoder, where the transmit signal u₁ of line 1 before gain scaling hasunit power. Furthermore, the precoder matrix P is scaled such that thediagonal elements are equal to 1, according to

P=H ⁻¹·diag(H ⁻¹)⁻¹.   (1.17)

The optimization is done with an objective function for all lines, whichis here the sum data rate. An additional constraint is introduced totake the limited modulation alphabet into account. There is an upperbound b_(max) and a lower bount b_(min), usually b_(min)=1 for the bitloading b per tone and line. This translates in a maximum required SNR

SNR_(max)=2^(b) ^(max) −1   (1.18)

and a minimum SNR

SNR_(min)=2^(b) ^(min) −1.   (1.19)

The maximum bit loading and the limit PSD is reformulated in a linearconstraint set of the form A·x=b. Instead of maximizing with respect tothe gain values s_(i), the squared gain values |s_(i)|² are used asarguments for the optimization problem

$\begin{matrix}{{\max\limits_{{s_{1}}^{2}\ldots {s_{L}}^{2}}{\sum\limits_{l = 1}^{L}\; R_{l}}}\begin{matrix}{{{s.t.{~~~}{\sum\limits_{i = 1}^{L}\; {{p_{li}}^{2}{s_{i}}^{2}}}} \leq {p_{\max}{\forall l}}} = {1\mspace{14mu} \ldots \mspace{14mu} L}} \\{{~~~}{{{s_{l}}^{2} \geq {0{\forall l}}} = {1\mspace{14mu} \ldots \mspace{14mu} L}}} \\{{~~~}{{\frac{{\; \left\lbrack H^{- 1} \right\rbrack_{ll}^{- 1}}^{2} \cdot {{s_{l}}^{2}}}{\sigma_{noise}^{2}} \leq {2^{b_{\max}} - {1{\forall l}}}} = {1\mspace{14mu} \ldots \mspace{14mu} {L.}}}}\end{matrix}} & (1.20)\end{matrix}$

The arguments which solve this optimization problem are the sum-rateoptimal scale factors.

Also, in embodiments, transmit spectrum variation may be employed.

With the coefficient recalculation as explained above, the transmitspectrum changes and some tones may violate the constraint from Eq.(1.13) as shown in FIG. 3.1. To allow low power modes on a per-symbolbasis without the re-computation of the transmit spectrum and avoidviolations of the transmit spectrum, the transmit spectrum must bechosen such that the constraints are not violated for any configuration.

If the scale coefficients of the matrix S are limited to be real andpositive, the output transmit powers diag(C_(tx)) are an increasingfunction of the scale factors. Therefore, it is possible to satisfy thetransmit power constraint for all possible configurations by configuringthe scale factors to the minimum scale factor of all configurations.

This is a stable configuration which is guaranteed to work for allpossible low power mode configurations which will occur duringoperation.

The term “quiet mode” referring to a line as used herein may refer to adeactivated line, a line in no-power mode, a line transmitting quietsymbols, a line transmitting idle symbols with no transmit power and thelike.

The above-described embodiments serve merely as examples and are not tobe construed as limiting. In particular, details or numerical valueshave been given for illustration purposes, but may be different in otherimplementations.

1.-36. (canceled)
 37. A method for providing a common bit loading tablefor multiple customer premises equipment (CPEs) in a network usingdiscontinuous operation for transmitting data in a vector crosstalkcancellation environment, the method comprising the steps of:determining in advance a set of bit loading tables for combinations oflines being in a transmit and quiet mode; and selecting a minimum gainfrom the plurality of bit loading tables and constructing the common bitloading table from the minimum gains.
 38. The method of claim 37,further comprising sending a message to the CPEs that the common bitloading table is required to be formulated.
 39. The method of claim 37,further comprising at least one of the step of, during a line joining,getting a report of SNR from CPE side or the step of avoiding influenceof crosstalk into SNR measurement.
 40. The method of claim 37, furthercomprising calculating an optimized transmit power spectral density thatsatisfies spectral mask constraints for all combinations of lines beingin a transmit or quiet mode.
 41. The method of claim 37, whereindetermining a set of bit loading tables or constructing the common bitloading table comprises calculating a minimum bit loading of allcombinations of lines being in a transmit or quiet mode and assigningbits with respect to a minimum signal to noise ratio.
 42. The method ofclaim 37, further comprising a downstream transmit spectrum shaping indiscontinuous operation.
 43. The method of claim 37, further comprisingdetecting performance critical configuration of lines being in atransmit or quiet mode.
 44. The method of claim 43, further comprisingexcluding critical configurations from constructing the common bitloading table or avoiding a critical configuration by transmitting idlesymbols on at least one line being in a quiet mode.
 45. The method ofclaim 37, further comprising precomputing coefficients for differentconfigurations of lines being in a transmit or quiet mode, estimatingrequired gains and determining a minimum gain to satisfy a predeterminedspectral mask.
 46. The method of claim 37, further comprising predictingrates for different configurations based on a first estimation and anoptimization following the first estimation.
 47. The method of claim 37,further comprising stopping discontinuous operations during a joiningprocess for calculation of an optimized power spectral density.
 48. Themethod of claim 37, further comprising, in case of a joining line,performing a final bit loading in a downstream direction based on signalto noise ratio with a minimum configuration power spectral density andall lines active.
 49. The method of claim 37, further comprisingperforming a signal to noise ratio measurement by detecting an averageerror power, based on sync symbol and/or based on an orthogonalsequence, and using the signal to noise ratio measurement for onlinereconfiguration in showtime.
 50. A device for providing a common bitloading table for multiple customer premises equipment (CPEs) in anetwork using discontinuous operation for transmitting data in a vectorcrosstalk cancellation environment, the device comprising a transceiver,and being adapted to: determine in advance a set of bit loading tablesfor combinations of lines being in a transmit and quiet mode; and selecta minimum gain from the plurality of bit loading tables and constructingthe common bit loading table from the minimum gains.
 51. The device ofclaim 50, the device being adapted to send a message to the CPEs thatthe common bit loading table is required to be formulated.
 52. Thedevice of claim 50, the device being adapted to calculate an optimizedtransmit power spectral density that satisfies spectral mask constraintsfor all combinations of lines being in a transmit or quiet mode.
 53. Thedevice of claim 50, wherein determining a set of bit loading tables orconstructing the common bit loading table comprises calculating aminimum bit loading of all combinations of lines being in a transmit orquiet mode and assigning bits with respect to a minimum signal to noiseratio.
 54. The device of claim 50, the device being adapted to detectperformance critical configuration of lines being in a transmit or quietmode.
 55. The device of claim 54, the device being adapted to excludecritical configurations from constructing the common bit loading table.56. The device of claim 54, the device being adapted to avoid a criticalconfiguration by transmitting idle symbols on at least one line being ina quiet mode.